The present invention is directed to an improved eddy current inspection test probe. In particular, the present invention is directed to an improved dual coil bobbin probe capable of detecting flaws in steam generator or other heat exhanger tubing with only one pass of the probe.
Dual coil bobbin probes have been used in standard non-destructive inspection of workpieces, such as tubing or pipes. The standard dual coil bobbin probe design usually consists of two closely spaced parallel coils (e.g. 0.032 inch) with the axes of the coils coincident with the long axis of the tube. The coils are wound about a non-conductor (insulator) normally made of plastic. Alternatively, an encircling coil may be used. An encircling coil is made in the same basic manner as the bobbin probe, except a hole is drilled coaxially through the test coil to permit the tube being inspected to pass through the encircling coil during testing. During operation, an oscillating current is passed through the probe coils, producing an alternating electromagnetic field around the coils. This field induces alternating eddy currents in any conductive material (e.g. tube walls) within the field. These alternating eddy currents produce a counter electromagnetic field which interacts with the voltage in the driving coils in a typical transformer action. The voltage on the driving coils is modified by the physical characteristics of the tube wall. These modifying characteristics include electrical conductivity, magnetic permeability, inside diameter, wall thickness, and the presence of defects in the tube wall. All of these characteristics except for tube wall defects are essentially balanced out by use of the dual coils which form opposing legs of an electrical bridge. This bridge becomes unbalanced in first one direction when a single coil senses a defect, and then becomes unbalanced in the opposite direction when the defect passes the other coil. The bridge output is monitored for these unbalanced states which indicate the presence of wall defects. This monitoring technique is well known and has become the basis for commercially available eddy current tube testing equipment.
A standard dual coil bobbin probe cannot reliably detect cracks in tube walls if the cracks are oriented in the circumferential direction. This is because the direction of the eddy current flow created with the standard probe is also in the circumferential direction. In order for the crack or defect to be detected, it must interrupt the flow of eddy currents which will alter the balance of the electrical bridge. For this interruption to take place, the flow of eddy currents must be at an angle to the direction of the crack or defect. Because the circumferential crack is not at an angle to the eddy current flow, there is no significant interruption of the eddy current, and therefore, no significant imbalance in the electrical bridge is recorded.
In the past, several probes have been designed to take advantage of the angular eddy current flow concept. One such probe is termed a "pancake probe." This probe has a single coil with an axis perpendicular to the surface of the tube. The pancake probe successfully enables the operator to detect defects in any orientation. However, its main disadvantage is that only a small segment of the workpiece (e.g. tube) being examined can be inspected in one pass. Accordingly, it is necessary to use multiple scans (i.e., eight scans) to inspect the entire workpiece.
Another probe design which has been under investigation is the 45.degree. angle dual coil bobbin probe, which possesses parallel coils at a 45.degree. angle from the transverse axis of the probe. This design has exhibited some success in detecting all types of defects, but it also requires multiple scans for complete inspection (i.e. four scans for the circumference of a tube).
An eddy current test probe which is not only multi-directional but also can scan the workpiece in a single pass has not been developed until the present time. The multi-directional angle probe of the present invention fulfills this long felt need in the field of non-destructive testing of workpieces by eddy current inspection techniques.